QVariant: use a typedef name when saving user types to QDataStream

Due to the way Qt 5 and 6 registered type names, they end up producing
different type names for the same content for a typedef. For example,
because Q_DECLARE_METATYPE can't manage a comma (it's a macro), users
are forced to write something like:

 using MyTypeMap = QMap<QString, MyType>
 Q_DECLARE_METATYPE(MyTypeMap)

Qt 5's Q_DECLARE_METATYPE's argument "MyTypeMap" was the only name we
knew about the type, so that's what got saved in the stream. However, Qt
6 QtPrivate::typenameHelper is much more clever and obtains the name
from the compiler itself, so it "sees through" the typedef and registers
"QMap<QString,MyType>" as the official type name.

If another library/plugin has a different typedef name for the same type
(e.g., StringTypeMap), it's indeterminate which type gets saved and will
even change from run to run (depends on the QHash order).

[ChangeLog][QtCore][QDataStream] If QDataStream is used with a
QDataStream::Version < Qt_6_0 to serialize a user type that was
registered via a typedef with the metatype system, the typedef's name is
used in the stream instead of the non-typedef name. This restores
compatibility with Qt 5, allowing existing content to read the same
QDataStreams; reading from older Qt 6 versions should not be affected.
(Note: if more than one typedef name is registered, it's indetermine
which name gets used)

Fixes: QTBUG-96916
Pick-to: 6.3 6.2
Change-Id: I2bbf422288924c198645fffd16a8d811aa58201e
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

3 files changed, 49 insertions, 6 deletions

diff --git a/src/corelib/kernel/qmetatype.cpp b/src/corelib/kernel/qmetatype.cpp
index 9494ff84f9..476234d7c5 100644
--- a/src/corelib/kernel/qmetatype.cpp
+++ b/src/corelib/kernel/qmetatype.cpp
@@ -1,6 +1,8 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2021 The Qt Company Ltd.
+** Copyright (C) 2021 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com
+** Copyright (C) 2021 Intel Corporation.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtCore module of the Qt Toolkit.
@@ -179,6 +181,42 @@ Q_GLOBAL_STATIC(QMetaTypeCustomRegistry, customTypeRegistry)
 
 } // namespace
 
+// used by QVariant::save(): returns the name used in the Q_DECLARE_METATYPE
+// macro (one of them, indetermine which one)
+const char *QtMetaTypePrivate::typedefNameForType(const QtPrivate::QMetaTypeInterface *type_d)
+{
+    const char *name = nullptr;
+    QMetaTypeCustomRegistry *r = customTypeRegistry;
+    if (!r)
+        return name;
+
+    QByteArrayView officialName(type_d->name);
+    QReadLocker l(&r->lock);
+    auto it = r->aliases.constBegin();
+    auto end = r->aliases.constEnd();
+    for ( ; it != end; ++it) {
+        if (it.value() != type_d)
+            continue;
+        if (it.key() == officialName)
+            continue;               // skip the official name
+        name = it.key().constData();
+        break;
+    }
+
+#ifndef QT_NO_DEBUG
+    QByteArrayList otherNames;
+    for ( ; it != end; ++it) {
+        if (it.value() == type_d)
+            otherNames << it.key();
+    }
+    if (!otherNames.isEmpty())
+        qWarning("QMetaType: type %s has more than one typedef alias: %s, %s",
+                 type_d->name, name, otherNames.join(", ").constData());
+#endif
+
+    return name;
+}
+
 /*!
     \macro Q_DECLARE_OPAQUE_POINTER(PointerType)
     \relates QMetaType
diff --git a/src/corelib/kernel/qmetatype_p.h b/src/corelib/kernel/qmetatype_p.h
index 16270bc374..2b533608be 100644
--- a/src/corelib/kernel/qmetatype_p.h
+++ b/src/corelib/kernel/qmetatype_p.h
@@ -233,6 +233,7 @@ static const QT_PREPEND_NAMESPACE(QtPrivate::QMetaTypeInterface) *getInterfaceFr
     case QMetaType::MetaTypeName:                                                                  \
         return QtMetaTypePrivate::getInterfaceFromType<RealName>();
 
+const char *typedefNameForType(const QtPrivate::QMetaTypeInterface *type_d);
 } //namespace QtMetaTypePrivate
 
 QT_END_NAMESPACE
diff --git a/src/corelib/kernel/qvariant.cpp b/src/corelib/kernel/qvariant.cpp
index b34da6ca18..b942ac50c7 100644
--- a/src/corelib/kernel/qvariant.cpp
+++ b/src/corelib/kernel/qvariant.cpp
@@ -1,7 +1,7 @@
 /****************************************************************************
 **
-** Copyright (C) 2020 The Qt Company Ltd.
-** Copyright (C) 2018 Intel Corporation.
+** Copyright (C) 2021 The Qt Company Ltd.
+** Copyright (C) 2021 Intel Corporation.
 ** Copyright (C) 2015 Olivier Goffart <ogoffart@woboq.com>
 ** Contact: https://www.qt.io/licensing/
 **
@@ -1314,13 +1314,17 @@ void QVariant::save(QDataStream &s) const
         }
     }
     const char *typeName = nullptr;
-    if (saveAsUserType)
-        typeName = d.type().name();
+    if (saveAsUserType) {
+        if (s.version() < QDataStream::Qt_6_0)
+            typeName = QtMetaTypePrivate::typedefNameForType(d.type().d_ptr);
+        if (!typeName)
+            typeName = d.type().name();
+    }
     s << typeId;
     if (s.version() >= QDataStream::Qt_4_2)
         s << qint8(d.is_null);
     if (typeName)
-        s << d.type().name();
+        s << typeName;
 
     if (!isValid()) {
         if (s.version() < QDataStream::Qt_5_0)